Abstract
Fatigue crack propagation at elevated temperatures (650 and 725°C) has been studied in a powder processed high strength nickel based superalloy, U720Li, in air and vacuum environments, using trapezoidal loading waveforms. Constant load (increasing ΔK) tests were used to generate da/dN-ΔK curves, while interrupted constant ΔK tests were used to study crack propagation paths and mechanisms. At 650°C in air there is little effect of any dwell at maximum load while at 725°C such a dwell causes a significant increase in the crack growth rate coupled with a transition from mixed mode to fully intergranular crack growth. In vacuum the growth rates were significantly lower, and the dwell caused little or no change in crack growth rate at 650°C, but an increase at 725°C. The crack path in vacuum changed from fully transgranular at 650°C to incorporate increasingly mixed mode growth at 725°C with dwell. The intergranular failure in vacuum was through creep cavitation of grain boundaries, while in air, static failure of oxygen embrittled boundaries dominated.
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